1. Field
The present invention relates generally to a disk drive and, in particular, to a head positioning control technique.
2. Description of the Related Art
In general, in a hard disk drive, head positioning control is performed to position a magnetic head to a target position (a target track or a target cylinder) on a disk medium. The magnetic head has a structure such that a read head comprised of, for example, a GMR (a giant magnetoresistive element) or a TMR (a tunnel MR) and write head are mounted on the same slider as a separate unit. The read head is comprised of a read only head for reading recorded data from a disk medium. The write head is comprised of a head for writing data onto the disk medium.
In the disk drive, the magnetic head is usually mounted on a rotary type actuator and, by the driving of the actuator, moved to a target position on the disk medium. There, the read head and write head in the magnetic head are positioned in a different position. Upon the recording and reproduction of data, therefore, it is necessary to adjust the moving of the magnetic head so that the magnetic head is positioned on the same track on the disk medium. That is, it is necessary to position the write head on a target track upon the recording of data and to position the read head on the target track upon the reproduction of data.
The amount of movement involved upon the adjustment of this movement is called as an offset. The offset value is uniquely determined according to the distance between the read head and the write head and the radial position of the magnetic head over the disk medium. That is, the offset value is calculated from the distance between the read head and the write head and the skew angle (differing dependent upon the radial position) of the actuator.
Generally, at the time of manufacturing a disk drive, the offset value is measured or calculated in track units and stored as a fixed value in a memory (a flash EEPROM, etc.) in the disk drive. A microprocessor (CPU) in the drive reads out the offset value from the memory at the time of performing head positioning and uses this value to adjust the head under the head position control. Or, the CPU performs an approximate calculation from the offset value on a plurality of reference measuring tracks and calculates an offset value of a corresponding track.
As a method of the manufacture of the disk drive, a disk medium with servo data recorded thereon by a dedicated servo track writer is incorporated in the disk drive. In such a disk drive, a so-called disk runout is generated in the incorporating process. The disk runout is synchronized with the rotation of a spindle motor which rotates the disk medium.
If there is any disk runout, the physical radial position of the magnetic head varies even on the same track during one rotation of the disk medium and there is also a variation in the offset involved. If, in the disk drive involving a disk runout, any head positioning control is made with the use of a fixed value, which is an offset value in the track unit, there exists a positioning portion at a position displaced toward the inner or outer circumference side of the track on the magnetic head during one rotation of the track on the magnetic head.
In a disk drive, generally, the track over which recording is performed by the write head is made wider than the track over which reproduction is performed by the read head. For this reason, even if some positional displacement of the read head occurs, no lowering of the quality of the reproduction signal occurs at a data read mode. In recent years, the difference between the recording track width and the reproduction track width is made smaller due to an increased tendency toward a higher recording density of the disk drive, so that an allowable positional variation in one circumference of the track is becoming smaller.
In order to solve such a problem, a method is proposed to correct an offset value in one circumference of the track (for example, see JPN PAT APPLN KOKAI publication No. 2005-216378). According to this method, an offset value in one circumference of a track is corrected with the use of the absolute value and phase parameters of a runout amount relative to each track. A method of measuring the runout amount relative to any track comprises, for example, of measuring the runout amount with an actuator fixed to a track of a measuring target.
As a method of measuring a runout amount, another method is also proposed which comprises, with a head fixed to a given radial position from a rotation center on a disk medium, measuring a runout amount based on a time interval of a clock mark reproduction signal which is output from the head (for example, see JPN PAT APPLN KOKOKU Publication 3198490).
As set out above, it is possible to, at a manufacturing step for example, measure a runout amount relative to any given track in the disk drive with a disk medium incorporated therein and correct an offset value on one circumference of the track based on the runout amount. That is, it is possible to store a correction offset value (sometimes generally referred to as offset information) on one circumference of each track in a memory of the disk drive.
If any external impact (shock), etc., acts on a disk drive (a product) during or after its shipment, there sometimes arises a positional displacement (sometimes referred to as a disk shift) of a disk medium fixed to a spindle motor. Upon the occurrence of a disk shift, there occurs a variation in a disk runout amount and phase, and any correct offset value which was stored in the memory of the disk will become invalid. When, in such disk drive, the magnetic head is positioned on a target track on the disk medium, it becomes impossible to effect a moving adjustment of the magnetic head in one circumference of the track. In the disk drive of a high recording density level, therefore, a reproduction signal becomes lower in quality at a data read mode and, in the worst case, the reproduction of the recorded data becomes impossible.